Influence of fiber reinforcement towards the physical
characteristics of low density polyethylene laminated
composites
S. Ravichandran1), *), A. Daniel Raj2), P. Balaji Visvanath2), Faruq Mohammad3), Hamad A. Al-Lohedan3), Suresh Sagadevan4), *)
DOI: dx.doi.org/10.14314/polimery.2020.6.4
Abstract: In recent years, the increased interest in the design and fabrication of lightweight polymer
composites with various combinations and stoichiometry is due to their enhancement of electrical, me-chanical, thermal, and biological properties compared to the properties of conventional materials. With that view, the present study deals with the effects of low density polyethylene composites (LDPE) re-inforced with epoxy resin, glass fiber, carbon fiber, and Kevlar towards the mechanical, thermal, and water absorption properties. The mechanical studies showed that the LDPE composite reinforced car-bon fiber has the best tensile properties compared to other composites and this can be mostly due to the proper bonding and associated interaction between the polymeric matrix and the bidirectional layer of the fibers. Also, the carbon fiber reinforced composite has superior properties of impart energy com-pared to the other composites and the non-reinforced ones and this is attributed to the crystalline nature of carbon fiber. Further studies of the thermal properties indicated that the retention of thermal stabil-ity for all the fiber-reinforced polymer composites, while the water absorption revealed a considerable increase in the weight of Kevlar fiber-reinforced composite. From the overall analysis, the enhanced properties of LDPE matrix reinforced fibers are linked to the morphological changes that occurred and are directly affected by the nature of the fiber.
Keywords: low density polyethylene composites, mechanical properties, thermal studies, water
absorp-tion study, epoxy resin, Kevlar fiber, glass fiber, carbon fiber.
Wpływ rodzaju włókien na właściwości fizyczne laminatowych
kompozytów polietylenu małej gęstości
Streszczenie: Zwiększone w ostatnich latach zainteresowanie projektowaniem i wytwarzaniem
lek-kich kompozytów polimerowych wynika z ich lepszych właściwości elektrycznych, mechanicznych, termicznych i biologicznych w porównaniu z cechami materiałów konwencjonalnych. Zbadano wpływ rodzaju wzmocnienia (włókno szklane, włókno węglowe i włókno Kevlar) na właściwości mechanicz-ne, termiczne i absorpcję wody laminatowych kompozytów polietylenu małej gęstości (LDPE) z ży-wicą epoksydową. Stwierdzono, że kompozyt LDPE z włóknem węglowym, w porównaniu z innymi kompozytami, wykazuje najlepszą wytrzymałość na rozciąganie, co może wynikać głównie z interakcji polimerowej osnowy z dwukierunkową warstwą włókien. Ponadto kompozyt ten ma większą zdol-ność przenoszenia energii niż pozostałe badane kompozyty, co można przypisać krystalicznej budowie włókna węglowego. Badania właściwości termicznych wykazały stabilność termiczną wszystkich kom-pozytów polimerowych wzmocnionych włóknami oraz znaczną absorpcję wody kompozytu wzmoc-nionego włóknem Kevlar.
Słowa kluczowe: kompozyty polietylenu małej gęstości, właściwości mechaniczne, badania termiczne,
badania absorpcji wody, żywica epoksydowa, włókno Kevlar, włókno szklane, włókno węglowe.
1) Sathyabama Institute of Science and Technology, Department of Physics, Chennai-600 119, India.
2) Sathyabama Institute of Science and Technology, Department of Mechanical Engineering, Chennai-600 119, India. 3) King Saud University, College of Science, Department of Chemistry, P.O. Box 2455, Riyadh, Saudi Arabia 11451. 4) University of Malaya, Nanotechnology & Catalysis Research Centre, Kuala Lumpur 50603, Malaysia.
The increased interest of research in polymers and plastic reinforcement both from the academic, as well as industrial, point of view is due to the possibility to form hybrid composites with integrated structures allowing for the advancement of applications. In that direction, the commonly applied materials for the reinforcement of polymer matrices include metallic nanoparticles, asbes-tos, paper, wood, glass fiber, carbon fiber, aramid fiber (polyaramids), jute fiber, kenaf fiber, etc. [1, 2]. Among all, significant development has been made with the use of fiber reinforcement as the incorporation of these mate-rials brought about concomitant changes in the volume fraction, architecture, and orientations to the final com-posite. Since the fibers are used as reinforcement to the polymers, it bears the load while the polymeric matrix functions to transfer the stress between the fibers, and thereby providing a barrier against the adverse envi-ronment that protects the fiber surface from mechani-cal abrasion [2, 3]. Out of many different types of fibrous reinforcements for polymer matrices, the incorporation of glass fibers as the reinforcement has the special advan-tages of enriching the mechanical properties (like tensile strength), resistance to thermal and chemical degrada-tions, electrically insulating properties and all can be pos-sible at a relatively low cost. But the glass fiber-reinforced composite suffers from the limitations of very high den-sity compared to the other fiber-reinforced composites like carbon and aramid. Carbon and aramid fibers, when used for the polymer reinforcement, have the advantages that include higher mechanical strength, lighter weight, higher dimensional stability, higher dielectric strength, resistance to corrosion, and flexibility to improve the material design. Similarly, the polymers used as adsor-bents for the loading of the filler include epoxy, polyester/ vinyl ester thermosetting plastic, phenyl formaldehyde resin, etc. In one study, Banakar and Shivananda [4] inves-tigated the mechanical properties of the carbon fiber reinforced epoxy resin composites like tensile and flex-ural strengths where the results indicated that the fiber orientation significantly influences the mechanical prop-erties of the laminated polymer composites. They found that the 90° orientation of fiber had superior properties of tensile and flexural strengths along with high load-bear-ing specimens, while the extension and deflection were maximum at 30° and minimum at 90° fiber orientation. A different study by Deogonda and Chalwa [5] showed that the mechanical properties of epoxy fiber composites were enhanced by glass fiber and with the addition of ZnS and TiO2 as the filler materials.
In general, the materials used for the aerospace, auto-mobile, and marine industries possess many advanced characteristics and, in that view, low density polyethyl-ene (LDPE) is a well-known commercial thermoplastic polymer that can offer many industrial applications like food and juice containers, waterproof surfaces, computer components, etc. The industrial applications of this LDPE material are due to its unique combination of properties
like insulating behavior for electrical devices, mechani-cally strong nature and at the same time the capacity for the maintenance of thermal resistance [6]. When the LDPE material is used for the composite formation with other reinforcements and/or fillers, the generated hybrid com-posites possess the additional characteristics of high spe-cific strength and modulus, excellent toughness against fractures, fatigue properties, good resistance to corro-sion, etc. [6–8]. This unique combination of properties, in particular, the high strength to weight ratio, makes LDPE polymers and its composites a very attractive platform for transport applications. Therefore, to take advantage of the inbuilt properties of LDPE material, the present study deals with the LDPE composites containing epoxy resin, and various reinforcements such as glass, carbon, and Kevlar fibers. For the formation of the composites, we followed the compression molding method where the formed hybrid composites were characterized thor-oughly and further studied towards the change in the mechanical, thermal, and water absorption properties.
EXPERIMENTAL PART Materials
The polyethylene matrix used in this study was low density polyethylene (LDPE) copolymer resin grade pro-duced by Sakthi fiberglass, Chennai, Tamilnadu, India. The density of the polymer was identified as 0.918 g/cm3,
melting point in the range of 114–133°C. Similarly, the epoxy resin (LY-556), hardener (HY-917), glass fiber (WRM 610 GSM), carbon fiber, and Kevlar fiber used as the reinforcement materials were also obtained from the same Sakthi fiberglass company.
Preparation of composite material
For the fabrication of fiber-reinforced polymer (FRP) composites, we followed the compression molding method and, for that, a mixture of LDPE and toluene sol-vent was taken in the ratio of 1 : 3 and heated to a ature of 190°C for 45 min. After cooling to room temper-ature, the reaction mixture was washed with degassed ethanol, centrifuged, the precipitate separated out, and the product dried to see the formation of solidified, smooth LDPE in its powdered form. The obtained powder was transferred to a round bottom flask and subjected to mixing on a magnetic stirrer with that of epoxy resin and fiber by taking in a ratio of 1 : 2 : 3 for LDPE powder, epoxy resin, and fiber, respectively. The mixture was allowed to stir for about 30 min or more until all the contents were blended well to form a homogeneous mixture and, after that, 10% of hardener was added to this mixture. A spacer of 5 mm thickness and 300 mm × 300 mm (inner dimen-sion) was mounted on the compression molding machine. Then layers of glass fibers were placed on one another by applying the LDPE and resin mixture. Both the sides
of the spacer were covered by polyethylene sheet coated with wax. A constant temperature of 80°C and a pressure of 0.02 MPa (3 psi) was maintained for 2 h for the FRP composite to be cured. The same procedure was adopted for the fabrication of LDPE-carbon and LDPE-Kevlar fiber composites too. The test specimen was prepared as per the ASTM standards (Fig. 1).
Methods of testing
Hardness measurements
The Charpy impact testing machine was used to deter-mine the amount of energy absorbed in fracturing the test piece of composite material. The absorbed energy is a measure of the toughness of the material and serves as a tool for studying ductile-brittle transitions. Hardness measurements are used to find the resistance of a mate-rial to plastic deformation.
Tensile test
The hybrid polymer composite material was fabricated in the required dimension using a suitable cutter and the edges finished by using emery paper for the tensile testing. The tensile test specimen is prepared as per the ASTM D638 standard and the test involves the mounting of material in a machine and then subjecting it to various tensions until it fractures. The tensile force is recorded as a function of an increase in gauge length. During the application of different tensions, the elongation of the gauge section is recorded against the applied force.
Flexural test
The three-point flexural test is used to measure the flex-ural strength that is reasonably significant with a tensile test and for the test, the specimen is rested between two knife supports and the opposite probe produces a flexural effect repeatedly until it breaks. The flexural specimens are prepared as per the ASTM D790 standards where the three-point flexure test is the most common for testing the flexural properties of the composite materials. The spec-imen deflection is measured by the crosshead position and the test results include flexural strength and displace-ment. The testing process involves the placing of the test
specimen in the universal testing machine and applying force to it until it forms fractures and breaks. The tests are carried out at 50% average relative humidity conditions.
Test of water absorption
Good matrix wetting of the fiber and adequate bond-ing of the fiber matrix can reduce the rate and quantity of water absorbed in the composite [9]. To determine the amount of water absorbed under the testing conditions, the water absorption tests of fiber-reinforced composites were done as per ASTM 570 by immersing the samples in distilled water at room temperature. The dried speci-men’s weight (w1) and after submerging the same mate-rial in distilled water for a 24 h period (w2) were recorded. The factors that affect water absorption characteristics like plastic-type, additives used, exposure temperature, and duration of soaking were also investigated. Due to the nature of the fibers, the water absorption of compos-ites containing various reinforcement fibers (glass, car-bon, and Kevlar) may be a limiting parameter for several composite applications. All the samples were dried com-pletely before being immersed in water until we confirm the constant weight on a four-digit balance. The percent-age of water consumption is calculated by Eq. (1) below.
WA (%) = [(w2 – w1)/w1] · 100 (1)
w1 – initial weight of dried specimen (g) and w2 – speci-men weight after N hours of water soaking (g).
RESULTS AND DISCUSSION Mechanical properties
To investigate the effects of type of reinforcement material on the LDPE’s internal characteristics, the syn-thesized composites were first subjected to a wide range of mechanical tests. In general, the tensile properties of the composites were affected by the type of materials, preparation methods, specimen condition and prepara-tion, and the percentage of reinforcement [10].
For the LDPE reinforced composites, the changes in the stress-strain behavior of fibers with respect to the rein-57.0 50.0 13.0 R 76.0 19.0 165.0
Fig. 1. Sample dimensions for the tensile strength tests
T a b l e 1. Ultimate tensile strength, maximum force, break-ing load, and energy absorption of fiber reinforced LDPE com-posites LDPE composites Ultimate tensile strength MPa Fmax kN Breaking load kN Energy absorbed J LDPE-epoxy resin laminate 120 7.2 0.05 0.01 LDPE-glass fiber 298 20.6 1.3 8.6 LDPE-carbon fiber 448 35.9 1.5 9.7 LDPE-Kevlar fiber 207 15.5 1.2 3.5
forcement material are shown in Fig. 2. Table 1 compares the tensile strength, breaking load (or breaking strength and the tensile strength at its break), energy absorption (maximum force applied), and impact energy of LDPE- -epoxy resin with that of other fiber reinforcements like glass, carbon, and Kevlar fiber.
From Fig. 2, remarkable differences can be observed for the mechanical properties of LDPE with different reinforcing materials and we found that the carbon fiber reinforced LDPE composite has the superior properties of tensile strength and high impact energy compared to the other composites. From Table 1, it can be clearly noted that the carbon fiber reinforcement of LDPE led to a substantial improvement of stiffness to 448 MPa, fol-lowed by the glass fiber with 298 MPa, and Kevlar fiber with 207 MPa. The comparison of variation of tensile strengths of LDPE-resin with that of other fiber-rein-forced composites shown in Fig. 2d indicates that the value obtained for carbon fiber is 3.5 times higher than the pure LDPE-resin system. The observation of this high value of tensile strength and breaking loads for the car-bon fiber reinforced LDPE composite satisfies the mini-mum requirement that any material needs to be main-tained so as to employ in the manufacturing of sheet molded components. Further, these enhanced properties for the carbon fiber reinforced LDPE composite against the other composites of resin, glass, and Kevlar fibers can be attributed to the formation of a hybrid crystal-line structure. Since the carbon fibers possess fully or halfway crystalline nature and, on composite forma-tion with the LDPE, this allows the proper associaforma-tion of bonding between the polymer matrices and the
bidirec-tional fiber layers [11]. Also, the carbon fibers that contain π electrons become covalently bonded with other carbon atoms of LDPE as they both are in the same phase, while van der Waals forces support the increase of interac-tions between the layers. Similarly, the LDPE-glass fiber composite showed moderate improvement in its tensile strength compared to the LDPE-resin and this is attrib-uted to the strength of silicon and other metals contained in the glass fibers. In addition, the strength of the LDPE--Kevlar fiber composite can be linked to the formation of an inter-chain bonds and associated strength acquired between the LDPE and fiber molecules by means of aro-matic stacking interactions. These stacking interactions produce considerable influence on the Kevlar fiber than the van der Waals interactions and chain length, where the strength achieved is less for the stacking interactions compared to the van der Waals and for that reason, we observe less strength for the Kevlar fiber than the carbon 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 40 80 120 160 200 240 280 240 200 160 120 80 40 0 Stress, N/mm 2 a) Stress, N /m m 2 Stress ,N /mm 2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Strain Strain Strain 0.35 0 100 200 300 400 500 b) c) d) 0 100 200 300 400 UT S in M Pa LDPE-Kevlar fiber LDPE-carbo n fibe r LDPE-glass fiber LDPE-epoxy
Fig. 2. Comparison of stress-strain graph for: a) LDPE-glass fiber, b) LDPE-carbon fiber, c) LDPE-Kevlar fiber, d) the variation of ten-sile strengths of the LDPE-resin with that of all other fiber reinforced composites
10 8 6 4 2 0 LDPE-epoxy LDPE-glass fiber LDPE-carbon fiber LDPE-Kevlar fiber Energy absorbed, J
Fig. 3. Comparison of impact energy for the LDPE-resin when glass, carbon, and Kevlar fibers are used as reinforcements
fiber loaded LDPE composites [12, 13]. Overall, from the tensile study, it is concluded that the LDPE-carbon fiber--reinforced composite possesses the high tensile strength and breaking load compared with other fiber reinforce-ments.
A composite’s impact strength is the ability of any material to resist fracture failures when the stress is applied suddenly at high speed and is interrelated with toughness. In a general term, it is the amount of energy required to break the specimen and as similar to the tensile strength these values also change with various parameters. Figure 3 and Table 1 (energy absorbed) show the impact values of LDPE composites formed with dif-ferent fibers like glass, carbon, and Kevlar.
From Table 1, the LDPE composites reinforced with glass (8.6 J) and carbon (9.7 J) fiber have the highest impact energy as against the Kevlar fiber (3.5 J). Compared to the impact strength of plain LDPE-resin (0.01 J), the reinforce-ment of LDPE with the fibers has a significant enhance-ment in the energy absorption and this is due to the compression applied during the fiber reinforcement that eliminates the void space in the final composite [14, 15]. Also, within the three fiber reinforcement materials, the observation of higher impact value for the carbon fiber as against the glass and Kevlar may be expected due to the generation of the homogeneous composite structure for the former composite, which further helped for the effective dissipation of stress/load and prevented the polymer from cracking [16]. Hence, the LDPE-carbon fiber reinforcement with long fibers and effective bond-ing strength at the interfaces, supported by the same phase of carbon atoms, forms a homogeneous structure that helps to transfer the applied stress/energy uniformly. Flexural strength is also linked to the mechanical prop-erties of the composite where it is a measure of bending strength or fractural strength of the material. In general for the testing, the specimens are tested by subjecting a cer-tain surface of the sample to the compression; however, failures can occur in tension at mid-span of the material.
The flexural strength for the three-point bend configu-ration for the prepared LDPE composites was calculated by Eq. (2)
σ = 3 FL/2bd2 (2)
where: σ – the strength under axial force, F – the maxi-mum load applied, L – the length of the supported span,
b – the specimen width, and d – the thickness.
Similarly, the maximum shear stress (τmax,force applied parallel to the surface to deform the material) at the neutral axis for the LDPE-fiber composites were calculated by Eq. (3)
τmax = 3 F/4bd2 (3)
The results of flexural strength and shear stress for the LDPE-fiber composites are shown in Fig. 4 and the respective data for both is tabulated in Table 2.
From the analysis of flexural strength data shown in Fig. 4a, all the composites exhibit values greater than the pure LDPE-epoxy laminated composite. As similar to the earlier tests of tensile and impact studies, the carbon fiber reinforced composites have a high flexural strength of 37 N/mm2, followed by the glass fiber with 22 N/mm2
and Kevlar fiber of 16 N/mm2. The observation of such
a higher flexural value for the carbon fiber composite supports the highly brittle nature compared to the other 40 35 30 25 20 15 10 5 0 Flexural strength ,N /m m 2 a) LDPE-epox y LDPE-Kevla r fibe r LDPE-glass fiber LDPE-carbon fiber b) LDPE-epoxy LDPE-Kevla r fibe r LDPE-glass fiber LDPE-carbon fiber 0 10 20 30 40 50 60 70 Max. shear stres s ( )· 10 ,N/mm Tmax -5 2
Fig. 4. Comparison of variations in the: a) flexural strengths, b) shear stress for the LDPE-fiber reinforced composites
T a b l e 2. Comparison of flexural strength, maximum shear stress, and hardness for the LDPE-fiber composites
Different LDPE composites Flexural strength N/mm2 Max. shear stress τmax · 10-5 N/mm2 Hardness values N/mm2 LDPE-epoxy resin 7.5 0.1 80 LDPE-glass fiber 22 0.3 107 LDPE-carbon fiber 37 0.4 67 LDPE-Kevlar fiber 16 62 111
fibers and pure LDPE. This may be related to the higher stiffness of carbon fibers and the ability of components within the LDPE-carbon fiber to redistribute across the fiber so as to withstand the load. As a result, the flexural strength of carbon fiber reinforced composite is 2.3 times greater than the Kevlar and 1.7 times greater than the glass fiber reinforcement. This remarkable improvement in the flexural strength of LDPE-carbon fiber compos-ite against the other fibers may be attributed to the C-C coupling effect of fiber with that of the LDPE polymer thereby making the composite more homogeneous. Turla
et al. [17]investigated the flexural strength of carbon and glass fiber reinforced epoxy based hybrid compos-ites where the results indicated higher values for carbon composites against glass ones. Based on the literature studies and from our data, it can be concluded that the flexural strength is dependent on the fiber material type and a significant enhancement in those properties can be achieved by reinforcing the polymers with that of fibers[18, 19]. In a similar way, the comparison of shear stress values [obtained using Eq. (3)] for the LDPE com-posites shown in Fig. 4b and Table 2, it was observed that the LDPE-Kevlar fiber composite has the highest value of 62 · 105 (N/mm2), while all other composites
exhib-ited negligible values of <0.5 · 105 (N/mm2). The higher
value of shear stress (tangential stress) indicates that
the LDPE-Kevlar composite maintains greater slippage capacity all along the planes in parallel to the applied force, while no such capacity is seen for the other fiber--reinforced composites.
Hardness is also one of the main characteristics of a polymer material where it determines the degree of the material’s deformation and is generally accepted as a significant property. This is defined as the indentation resistance and determined by measuring the indenta-tion’s permanent depth. In Wilson hardness testing of LDPE-fiber composites, a load was applied from 10–60 kg to the specimen for a Dwell time (time spent in the same position) of 5 s and the hardness values were measured. The results obtained from the Wilson hardness test with our LDPE-resin polymer composites of different fiber reinforcements are also tabulated in Table 2. From the comparison of values, it indicates that the hardness of the composites is varied with respect to the nature of fiber reinforcement. The composites with glass and Kevlar fiber have high hardness values because of the alignment and the length of the fibers, which lead to the contribution of the loads along the fibers. However, the LDPE-carbon fiber composite maintains a lower value of hardness and because of a homogeneous mixture that allowed for the improved interaction among the groups of LDPE matrix and the carbon fiber [20, 21].
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 LDPE--epoxy LDPE-glass fiber LDPE-carbon fiber LDPE-Kevlar fiber a) Absorption coefficient [ ( )] W A t 50 m b) 10 m c)
Fig. 5. Comparison of the: a) water absorption coefficient of various LDPE-fiber composites, b) SEM fractional section of LDPE-epo-xy resin composite, c) LDPE-carbon fiber-reinforced composite
Water absorption studies
Water absorption may adversely affect several mechan-ical properties and can also increase the moisture content in fiber walls and interphase regions of the fiber matri-ces and so the test of water absorption is a very impor-tant one for determining the outdoor work potential of fiber-reinforced composites. These performances of the composites may suffer if they are exposed to vari-ous environmental conditions and the physical proper-ties of the fiber material like the changes in surrounding temperature, fiber size and volume fraction, fiber type and orientation, total exposed area, diffusivity, interface bonding, void space, viscosity and surface protection, water-matrix interaction, etc. In general, a good matrix wetting of the fiber and adequate bonding of fiber with that of a polymer matrix can reduce the amount and rate of water absorbed in the interphone region of the com-posite [22, 23]. Therefore, to investigate the extent of inter-action between the epoxy resin, fiber, and LDPE groups, the LDPE-epoxy resin and LDPE-fiber reinforced com-posites were tested for water absorption and the results are provided in Fig. 5a. From the data showed in Fig. 5a, it is indicated that the composite with Kevlar fiber has high water absorption, while the composite with carbon fiber
has a lower water absorption coefficient and this value is even less than the LDPE-epoxy resin. The observation of lower water absorption for the LDPE-carbon fiber-reinforced composite may be attributed to the absence of voids and cavities in the composite. However, for the Kevlar fiber composite, there may be much space at the interface region between the fiber and matrix which sup-ported a higher water intake.
To confirm the water absorption phenomenon, SEM studies were performed using the LDPE-epoxy resin and the LDPE-carbon fiber-reinforced composite where the results are shown in Figs. 5b, 5c. The SEM image of the epoxy resin sample (Fig. 5b) indicated a heterogeneous sponge like morphology, and one can clearly identify the LDPE and epoxy resin in two different phases. However, for the SEM of the carbon fiber composite (Fig. 5c), the homogeneity of the polymer matrix along with the fiber reinforcements is maintained, in addition to the lack of spongy nature. Further, the physical observations of LDPE-epoxy resin powder indicated for the soft and spongy nature, while the LDPE-carbon fiber sample for rigid and stiff behavior. From the morphological analysis, it can be confirmed that the reinforcement of the LDPE matrix with that of carbon fiber brings some changes to the physical structure and that the formed homogeneous
Fig. 6. TG and DTA analysis of: a) LDPE with resin composite, b) LDPE-glass fiber, c) LDPE-carbon fiber, d) LDPE-Kevlar fiber re-inforced composites 5 4 3 2 1 0 -50 0 50 100 150 200 Temperature, °C 1.732 mg 71.8°C 1.816 mg 0.41% 0.007 mg 99 98 97 96 DT A, V TG, % 95 1.82 1.81 1.80 1.79 1.78 1.77 1.76 1.75 1.74 1.73 TG, mg 71.8°C 76.0°C 1.809 mg 1.808 mg 30.5°C 1.816 mg 4.13% 4.76% 145°C 1.808 mg 0.075 mg 145°C 65 70 75 80 85 90 TG, % 95 100 35 30 25 20 15 10 5 0 -5 -10 -15 -20 DT A, V -400 -200 0 200 400 600 800 Temperature, °C 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 TG, mg 697°C 1.831 mg 359°C 6.98 V 36.1% 0.885 mg 697°C 2.717 mg 31.9% 170°C 2.774 mg 0.054 mg 1.9% 49°C 2.774 mg170°C 232°C2.72 mg 2.717 mg 1000 1200 20 30 40 50 60 70 80 90 100 TG, % -20 -10 0 10 20 30 40 DT A, V 1.4 1.2 1.0 0.8 0.6 0.4 TG, mg -500 0 500 1000 1500 Temperature, °C 79.9% 1.349 mg 978°C 1.028 mg 70.3% 195°C 0.083 mg 1.113 mg 73°C 1.443 mg195°C 1.36 mg 1.349 mg 978°C 0.321 mg 100 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 TG, mg 20 18 16 14 12 10 8 6 4 2 0 DT A, V 95 90 85 80 75 70 65 60 55 TG, % -200 0 200 400 600 Temperature, °C 169.3°C 2.653 mg 341.2°C 9.26 V 13.5% 1.796 mg 169.8°C 1.119 mg 38.6% 1.169 mg 156.1°C 213.6°C 1.119 mg 156.1°C 3% 38.5°C 1.608 mg a) b) c) d)
composite allows for the absorption of only limited water content.
Thermal analysis
TGA and DTG are used to characterize the decompo-sition and thermal stability under various conditions and to analyze the degradation kinetics of the physi-cochemical processes of the samples. Figure 6 shows the TGA and DTG analysis for the LDPE-epoxy and LDPE-fiber reinforced composites where, for the test-ing, the initial sample weights were taken in the range of 2–5 mg and were heated at a heating rate of 20°C/min from room temperature to higher levels. The weight loss usually indicates the rate of decomposition of polymers and often provides mechanisms for degradation and other important information related to physicochemical changes in different environments [24]. From Fig. 6a, the TGA of LDPE-epoxy resin represents a dynamic weight loss profile and for the LDPE-fiber composites, the major degradation occurrs in two steps, one due to fiber deg-radation while the other from polymer degdeg-radation. We observed the initial decomposition of material in the temperature range of 200–300°C, where the weight losses appeared to be very less and may be due to the loss of adsorbed water/moisture. The second weight loss stage occurred in the range of 400–750°C and can be linked to the degradation of the polymer present in the composites. From the TGA curves of the LDPE compos-ites, we observed significant weight losses in the range of 170–800°C with the glass-reinforced fiber (Fig. 6b), 195–978°C for the carbon fiber (Fig. 6c), and 156–470°C for the Kevlar fiber (Fig. 6d). The complete decompo-sition of the composite material was observed around 1000°C in LDPE with carbon fiber, at 800°C for the glass fiber, and 470°C for the Kevlar fiber-reinforced com-posites. Overall, we observed a 65.8% residue at 697°C with the glass fiber, 22% at 978°C for the carbon fiber, and 57% residue at 469°C for the Kevlar fiber-reinforced composites. This indicates that the LDPE-carbon fiber composite has significant thermal stability as it can be able to withstand a heating temperature of 978°C with 22% residual weight.
In a similar way, with the DTA analysis of LDPE-fiber reinforced composites, we observed polymer degrada-tions around 359°C for the glass fiber (Fig. 6b), 350°C for the carbon fiber (Fig. 6c), and 341°C for the Kevlar fiber (Fig. 6d) reinforced composites. Since the LDPE is a crys-talline polymer and it is common to observe such deg-radation peaks linked to phase changes, the obtained results can be compared, and are consistent, with litera-ture studies [25, 26]. The curve also shows that thermal degradation only started to occur after absorbing cer-tain amounts of heat energy by the composite materials. Under the heat absorbed conditions, the degradation pro-cesses are initiated and this helps the breakdown of the fiber structure and polymer matrix by causing ruptures
in the molecular chain. Rezaei et al.[27]and Zabihzadeh
et al.[28] reported similar results of thermal degradation of fibers and polymer matrices occurring due to molecu-lar chain ruptures. Thus, from the comparison of peaks found in the TGA and DTG curves, the physical structure of the LDPE polymer matrix seems to change after fiber reinforcement and this significantly affected the thermal degradation process of the composites.
CONCLUSIONS
We studied the thermal, mechanical, and water absorp-tion characteristics of LDPE matrices with epoxy resin and other composites reinforced with glass, carbon, and Kevlar fibers. From the studies of mechanical proper-ties, we observed a significant improvement in the fiber-reinforced composites compared to the LDPE with epoxy resin. Within the fiber-reinforced composites, the LDPE- -carbon fiber has high tensile strength and impact energy, while the LDPE-Kevlar composite has the highest hard-ness only. On testing the behavior of composites towards water absorption, we found that the LDPE-Kevlar fiber is very sensitive to swelling as it showed the maximum absorption coefficient and can be linked to the nature of the fiber and voids in the polymer matrix and least for the LDPE-carbon fiber. Further, the thermal studies con-firmed that with the reinforcement of carbon fiber, the thermal stability of LDPE composite increased signifi-cantly. Overall, as compared to the non-reinforced poly-mer matrices, the reinforcement of the LDPE matrix with fiber has significant properties of mechanical, thermal, and water absorption and within the different fibers, the carbon fiber reinforcement is the most influential one due to the modifications occurring in its morphology. Further, these composites can be incorporated directly for various industrial applications by taking advantage of the lim-ited/no water absorption property of LDPE-carbon fiber composite and high water absorption of LDPE-Kevlar fiber, in addition to the thermal and mechanical stabil-ity of LDPE-carbon fiber. Finally, the enhancement in the mechanical and other physical properties due to the fiber reinforcement due to morphological changes can be explored to form composites with other polymer matri-ces that have a significant effect on the industrial product development.
ACKNOWLEDGMENTS
This work was financially supported by the University of Malaya Research Grant (RU001-2018, RU001-2019). The authors also acknowledge funding of this work by the Research Supporting Project (RSP-2019/54), King Saud University, Riyadh, Saudi Arabia.
REFERENCES
[1] Rajak D.K., Pagar D.D., Kumar R., Pruncu C.I.: Journal
of Materials Research and Technology 2019, 8, 6354.
[2] Adeniyi A.G., Onifade D.V., Ighalo J.O., Adeoye A.S.:
Composites B Engineering 2019, 176, 107305.
https://doi.org/10.1016/j.compositesb.2019.107305 [3] Gnaba I., Legrand X., Wang P., Soulat D.: Journal of
Industrial Textiles 2019, 49, 71.
https://doi.org/10.1177/1528083718772299
[4] Prashanth Banakar, Shivananda H.K.: Journal of
Mechanical Civil Engineering 2012, 1 (2), 15.
http://dx.doi.org/10.9790/1684-0131518
[5] Deogonda P., Vijaykumar Chalwa N.: International
Journal of Scientific Engineering and Research 2013, 1 (4), 6.
[6] Jordan J.L., Casem D.T., Bradley J.M. et al.: Journal of
Dynamic Behavior of Materials 2016, 2, 411.
http://dx.doi.org/10.1007/s40870-016-0076-0
[7] Khanam P.N., AlMaadeed M.A.A.: Advanced
Manufacturing: Polymer and Composites Science 2015, 1, 63.
https://doi.org/10.1179/2055035915Y.0000000002 [8] Dintcheva N.Tz., Filippone G., Arrigo R., La Mantia
F.P.: Journal of Nanomaterials 2017, ID 3549475. https://doi.org/10.1155/2017/3549475
[9] Wang Q., Liu C., Chen Y.: Plastics, Rubber and
Composites Macromolecular Engineering 2001, 30, 363.
https://doi.org/10.1179/146580101101541741
[10] Nassar A., Nassa E.: Nanoscience and Nanoengineering
2013, 2, 89. http://dx.doi.org/10.13189/nn.2013.010201
[11] American Standard of Testing Material (ASTM): “Standard test method for tensile properties of rein-forced thermosetting plastics using straight-sided specimens”, ASTM Int West Conshohocken, Pa, USA, pp. 32–37, 2004.
[12] Rahmani H., Mahmoudi Najafi H.S., Ashori A.:
Journal of Reinforced Plastics and Composites 2014, 33,
733.
http://dx.doi.org/10.1177/0731684413518255
[13] Atuanya C.U., Edokpia R.O., Aigbodion V.S.: Results
in Physics 2014, 4, 88.
http://dx.doi.org/10.1016/j.rinp.2014.05.003
[14] Banakar P., Shivananda H.K., Niranjan H.B.:
International Journal of Pure and Applied Sciences and Technology 2012, 9, 61.
[15] Etcheverry M., Barbosa E.S.: Materials 2012, 5, 1084. https://doi.org/10.3390/ma5061084
[16] Arbelaiz A., Fernandez B., Ramos J.A., Retegi A.:
Composites Science and Technology 2005, 65, 1582.
http://dx.doi.org/10.1016/j.compscitech.2005.01.008 [17] Turla P., Kumar S.S., Reddy H.P., Shekar K.Ch.:
International Journal of Engineering Research and Technology 2014, 3, 394.
[18] Amuthakkannan P., Manikandan V., Winowlin Jappes J.T., Uthayakumar M.: Materials Physics and
Mechanics 2013, 16, 107.
[19] Jang J., Han S.: Composites Part A 1999, 30, 1045. [20] Joseph P.V., Joseph K., Thomas S.: Composites Science
and Technology 1999, 59, 1625.
https://doi.org/10.1016/S0266-3538(99)00024-X
[21] Al- Hasani Emad S.: Engineering and Technology 2007,
25, 988.
[22] Nitz H., Reichert P., Romling H.,Mülhaupt R.:
Macromolecular Materials and Engineering 2000, 276-277, 51.
h t t p s : / / d o i . o r g / 1 0 . 1 0 0 2 / ( S I C I ) 1 4 3 9 -2054(20000301)276:1<51::AID-MAME51>3.0.CO;2-Z [23] Huner U.: Advances in Science and Technology Research
Journal 2015, 9, 1.
https://doi.org/10.12913/22998624/2357
[24] Prabhu T.N., Demappa T., Harish V.: Journal of Applied
Chemistry 2012, 2, 39.
[25] Chung D.D.L.: Thermochimica Acta 2000, 364, 121. https://doi.org/10.1016/S0040-6031(00)00631-6
[26] Liufu S.C., Xiao H.N., Li Y.P.: Polymer Degradation and
Stability 2005, 87, 103.
https://doi.org/10.1016/j.polymdegradstab.2004.07.011 [27] Rezaei F., Yunus R., Ibrahim R.A.: Materials and
Design 2009, 30, 260.
https://doi.org/10.1016/j.matdes.2008.05.005
[28] Zabihzadeh S.M., Dastoorian F., Ebrahimi G.: Journal
of Reinforced Plastics and Composites 2010, 12, 1814.
https://doi.org/10.1177/0731684409335455
